Aluminum alloy



Patented Mar. 30, 1937 UNITED STATES PATENT OFFlCE ALUMINUM ALLOY Walter Bonsack, Cleveland Heights,

G. Frost, Cleveland, Ohio, assignors to and John G.

The National Smelting Company, Cleveland, Ohio, a

corporation of Ohio 6 Claims.

This invention relates to alloys, and more particularly aluminum base alloys having a low thermal expansion and high thermal conductivity, this application being a continuation in part of our co-pending application Serial No. 43,635, filed October 4, 1935.

In the making of cast articles, such as bear- -ings, heat transfer elements, cylinder heads and similar articles utilized in internal combustion engines and motor constructions, it is often desirable to make such articles from aluminum and alloys thereof because of their low specific gravity.

While aluminum and aluminum alloys in general have a high thermal expansion, some aluminum alloys have been developed with a considerably lower thermal expansion, making them more desirable for use in the manufacture of castings for motor parts.

It has been found, however, that, in addition to having a low thermal expansion, aluminum alloys for use in making motor part castings should also have a high thermal conductivity, as well as good mechanical and physical properties, such as ductility, machinability and suitable strength at low temperatures or at elevated temperatures occurring in internal combustion engines.

It is, therefore, an object of this invention to provide a light weight aluminum base alloy having a comparatively low thermal expansion and a comparatively high thermal conductivity, and also having suitable mechanical and physical properties for the casting of engine parts and other similar articles requiring similar properties.

Another object of this invention is to provide such-an aluminum base alloy which is readily machinable, which may be readily cast in the usual type of molds, and which has a fine, homogeneous grain structure, and which may be subjected to elevated temperatures without causing loss of strength.

While certain aluminum-silicon alloys are known to have a relatively low thermal expansion and relatively high thermal conductivity as compared to other aluminum alloys, it is necessary with these aluminum-silicon alloys to have the other desirable properties of ductility, machinabiJity and good bearing qualities to make them more suitable for the production of bearings,

heat transfer elements,uor engine parts, and it is an object of this invention to provide such an aluminum-silicon alloy with such desirable mechanical properties and still maintain the relatively low thermal expansion and relatively high thermal conductivity of these allo s.

It has been discovered that if tin is added in proper proportions to aluminum base alloys containing silicon it will increase the thermal conductivity without materially affecting the thermal expansion of the alloy. Tin also improves the machinability of the alloy, and provides better bearing qualities in castings made therefrom.

Castings, such as bearings, heat transfer elements, or engine parts, which are subjected to elevated temperatures may, therefore, be produced from an alloy of aluminum, silicon, and tin and have excellent properties for the manufacture of articles of this nature.

In preparing our improved aluminum base alloy, silicon is used as the predominating ingredient and may be present in amounts ranging from 9% to 15%. The higher percentages of silicon are more effective in reducing the thermal expansion of the alloy. It is desirable, however, to avoid the use of too large a proportion of silicon, since it has a tendency to segregate out in large crystals, which makes the machining of the metal more difficult. It is, therefore, preferable to use silicon in an amount from 10% to 13%.

If the percentage of tin is too high it has a Tin is an important addition to the alloy in order to provide better thermal conductivity and/ bearing qualities, as well as to improve its machinability. In order to obtain these properties, tin may be present in the amount of. .3% to 3%, and preferably in an amount from 5% to 1.5% or 2%. Tin, if present in too large amount, has a tendency to segregate and it is not desirable to have more than about 3% tin in the alloy.

Copper may be present in the alloy in small amounts, for in amounts such as .6% or less it does not seem to materially affect the thermal expansion of the alloy, and it is, therefore, not necessary, in making the alloy, to exclude base metals containing small amountsof copper. If copper is present in the alloy in the amount of approximately 1% or more, it has the effect of substantially reducing the thermal conductivity of the alloy.

If relatively high thermal conductivity of the Chromium, manganese, nickel, cobalt and titanium tend to reduce the thermal conductivity of aluminum-silicon alloys. These metals also have a tendency to form compounds with aluminum 5 at a high melting point, and such compounds tend to segregate from the molten metal before solidification occurs. When a relatively high thermal conductivity of the alloy is desired, the above metals should not be present in an amount suificient to materially reduce the thermal conductivity, and, if present at all, should only be present in such minimum amounts as are practicable in the commercial manufacture of the alloy, or as will not reduce the thermal conductivity beyond the limit desired for the alloy.

Iron also has the tendency to reduce thermal conductivity, but iron, as is well known, is usually present as an impurity in aluminum or aluminum alloys. However, it should not be present in an excessive amount, such as 1% or more.

Many types of castings where rapid heat transfer is desired, such as for compressors, bearings, motor parts and other castings which are subjected to elevated temperatures, or even for heat transfer parts of a refrigerator where the castings are used at low temperatures, can be advantageously made of an alloy of aluminum, silicon and tin.

Since the tin in the alloy has the effect of increasing machinability and improving bearing qualities without materially aifecting tensile strength or thermal expansion, it is desirable to employ an alloy for bearings having a higher tin content than is necessary for castings such as cylinder heads or heat exchange pipes.

As illustrations of our improved alloys the following specific examples are given:

An alloy containing 12 or 13% silicon, 1% tin and the balance aluminum and minor impurities was chill cast and aged for 15 hours at 200 C. Upon being tested it was found to have a thermal expansion of approximately 20 10 inch per inch per degree centigrade between a temperature range of 20 and 100 C., and it also had thermal conductivity of about .36 calorie per square centimeter per second at 30 C. With the same alloy, with the exception that the amount of tin was 2%, instead of 1%, the thermal' conductivity was approximately .38 calorie per square centimeter per second at 30 C., and the thermal expansion was the same. With an alloy containing 12 to 13% silicon and .5% tin the thermal conductivity was approximately .34 calorie per square centimeter per second at 30 C. and thermalexpansion was approximately the same as given in the above examples.

The 'alloy containing 12 to 13% silicon and 1% tin when chill cast and aged for 8 hours at 175 C. was found to have a tensile strength of 21,000

pounds per square inch, an elongation of 3.5% in 2 inches, a hardness of 47 Rockwell E; The above alloy with 2% tin instead of 1% and similarly aged was found to have a tensile strength of 20,000 pounds per square inch, an elongation of 3% in 2 inches, and a. hardness of 47 Rockwell E- An alloy containing about 10% silicon and 1% tin when chill cast and aged for 8 hours at 175 C. has a thermal conductivity of about .37 calorie per square centimeter per second at 30 C., and has a thermal expansion of approximately 21 10 inch per inch per degree centigrade between a temperature range of 20 and C. By annealing such an alloy the thermal conductivity may be increased to about .42 calorie per square centimeter per second at 30 C. With the same alloy having .5% tin instead of 1% the thermal conductivity was about .36 calorie per square centimeter per second at 30 C., and thermal expansion was about 21.4 10- inch per inch per degree centigrade between a temperature range of 20 and 100 C.

Alloys of aluminum, silicon and tin having percentages of silicon and tin within the limits herein set forth have a relatively low coeificient of expansion and a relatively high thermal conductivity. Alloys having higher silicon percentages have lower coefficients of expansion and lower thermal conductivity. When the percentage of silicon is reduced, however, the coefiicient of expansion is increased. Also percentages of silicon within the range specified may be desirable in alloys in which the quality of high thermal conductivity is desired and a somewhat higher coefiicient of expansion is permissible.

As previously stated, our invention contemplates that copper may be present in small aniounts in the alloy and it is also to be understood that minor impurities may be present without departing from the scope of the invention set forth in the claims.

Furthermore, it will be understood that the present invention is not limited to the specific details set forth in the foregoing examples, which should be construed as illustrative, and not by way of limitation, and in view of the numerous modifications which may be effected therein without departing from the spirit and scope of this invention, it is desired that only such limitations be imposed as are indicated in the appended claims.

What we claim is:

1. An aluminum base alloy having a low coeflicient of expansion and high thermal conductivity comprising silicon 9 to 15%, tin .3 to 3%, and the balance substantially all aluminum.

2. An aluminum base alloy having a low coeflicient of expansion and high thermal conductivity comprising silicon about 10 to 13%, tin .3 to 3%, and the balance substantially all aluminum.

3. An aluminum base alloy having a low coeflicient of expansion and high thermal conductivity comprising silicon 9 to 15%, tin about .5 to 2%, and the balance substantially all aluminum.

4. An aluminum base alloy having a low coefiicient of expansion and high thermal conductivity comprising silicon about 10 to 13%, tin about .5 to 2%, and the balance substantially all aluminum.

5. Aluminum base alloy having a low coeflicient of expansion and high thermal conductivity comprising silicon 9 to 15%, tin about 1% and the balance substantially all aluminum.

6. A chill casting formed from an aluminum base alloy having a low coeflicient of thermal expansion and high thermal conductivity comprising about 10 to 13% silicon, .5 to 1.5% tin, and the balance substantially all aluminum.

WALTER. BONSACK. JOHN G. G. FROST. 

